Automatic frequency control system



M. G. CROSBY" AUTOMATIC FREQUENCY CONTBQL SYSTEM Dec. 29, 193e.

Filed June 13, 1932 I5 Sheets-Sheet l R05 BY l mvENToR MURRAY @..c

BY M

ArroRNlw Dec- 29, 1936- M. G. cRosBY AUTQMATIC FREQUENCY CONTROLSYSTEM s sheets-sheet 2 Filed June 13, 1932 O ga 105713 1 a/07 vmvsmon wz; MURRAY e. cnossv BY ATTORNEY Dec. 29, 1936.

M G. cRosBY AUTOMATIC FREQUENCY CONTROL SYSTEM Filed June 13, 1952 3 Sheets-Sheet 5 I INVENToR MURRAY 6. CROSBY l BY MW ATTORNEY www@ Prasadpee. 29,1936

Aulsu'rlan s-'ni'rlzs PATENT OFFICE AUTOMATIC FREQUENCY CONTML l SYSTEM Murray (El.v Crosby, Riverhead, N. Y., assignor to I Radio of Delaware Corporation of America, a corporation applicaties :uns 13, 1932, sensi No. 616,803 3o claims. (ci. 25o-zo) frequency control means of the present inventiony is applicable to receivers of frequency or phase or l amplitude modulated oscillations. The automatic frequency control maybe readily applied to new ceivers or to receivers known lheretofore in the l heretofore in the art. The frequency control means known heretofore in the art have been used to some extent in frequency modulated signal receivers. Frequency control means have not been used heretofore in crystal filter receivers `adapted to receive phase or amplitude modulatedsignals.

The filter circuits used in phase or frequency or amplitude modulated signal receivers in general used heretofore are relatively broad and 2 5 therefore the frequency control lmeans known heretofore were suiciently efiicient to maintain lthe tune of the receiver fairly close to the fre- I quency of the signal and, since the filters were broad, operation of these 'receivers was comparatively satisfactory.

In the novel receiver of the present invention the carrier wave is ltered by meansof a highly selective crystal lter such as disclosed more in detail in Hansel] U.- S. application #564,770 flied Sept. 24, 1931, now Patent No. 2,001,387, May 14, 1935 and in Hansell' U. S. application #611,050 filed May 13, 1932, now Patent No. 1,999,902, April 30, 1935. Since this filter is crystal controlled and highly selective the band of frequencies which it will pass is very vnarrow and therefore'frequency control means as known heretofore in the art will be useless to control the frequency of a receiver in which the crystal filter referred to above is incorr porated. l

4* The automatic frequency control of the present invention is suiiiciently sensitive and accurate to maintain the tune of the signals constant, or approximately constant, so that they are at all times 50 passed by the selective crystal control lter cirist The absorbed signals are fed to a mst detector,

Frequency -control means have been known which is coupled with a high frequency generator. The intermediate frequency signals appearing in the output circuit of the rst detector are fed by way of a crystal lter or of an artificial transmission line or wave lter to a limiter device, 5 and from there to detecting means, from which the signals are utilized. I'he automatic frequency control means comprises a thermionic tube having its anode coupled to the frequency determining circuits of the oscillator and its input circuit con- 1o nected with the output circuits of two thermionic detector tubes to utilize the differential currents resulting therein when the input circuit to the two detector tubes is fed by a combination of the ltered and unfiltered circuit to control the fre' l5 quency of the high frequency oscillator of the superheterodyne arrangement. If phase or amplitude modulated signals are to be received the signals from the intermediate frequency amplifier are passed through the crystal filter 20 i circuit referred to above. If the signals to be rey ceived are frequency modulated the signals api pearing in the output circuit .of the intermediate frequency amplifier are passed through an artifiv cial line or long transmission line or tuned lter 25 circuit, of thetype described more in detail in Crosby United States application No. 618,154 led Jun'e 20, 1932.

'I'he rectiers connected with the utilization circuit include means for connecting the anode 30 circuits thereof in push-pull relation or in parallel so that phase or amplitude modulated sig` nals may be detected. In a mcdication the rec` tiiiers 'connected with the utilization circuit also serve to provide the differential currents which 35 operate through the thermionic control tube to determine in part the frequency of the oscillations generated by the generator connected with the first detector.

The accomplishment of this phase-synchronism 4o enables the use of circuits where the ltered circuit is recombined with the signal in such a man- \ner as to produce a more perfect detection of the signal wave as well as reduction of certain kinds of interference.`

Another apparent iadvantage of this receiver is that the iiltered carrier oscillations may be limited to a constant value so as to effect an automatic volume control in the present invention. Y

The recombination of the filtered carrier with the unfiltered signal also cancels out the second harmonic square law detector distortion so that distortions due to fading are minimized.

Heretofore the reception of phase modulated oscillations is' accomplished by means of -a re- 55 ceiver of' frequency modulated oscillations or signals which has in its output circuit amplitude correcting means. In some cases reception of wherein automatic tuning means is included,

eliminates these disadvantages, thereby making phase modulated oscillation signalling a more valuable signalling method.

The novelty in the type of frequency control, herein described, lies in the fact that phase detection of the signal is utilized to provide the controlling energy. The prior art includes systhe local oscillations, the production of which will be described more in detail hereinafter, produces in the output circuit of the rst detector 2 an intermediate frequency which is transmitted to the intermediate frequency amplifier 4. The details of the radio frequency amplifier and of the first detector, and of the intermediate frequency amplifier form no part of the present invention, and in order to reduce the length of the present specification such units will not be. described'in detail here. It will, however, be noted that the radio frequency amplifier, the first detector and the intermediate frequency amplier may be of any of the types known in the art today.

'I'he output circuit 'of the intermediate frequency amplifier 4 is connected with a. line 6 and with the movable elements 8 of a doubl\pole, double throw switch 1. Two of the terminals of this double pole, double throw switch are contems in Awhich differential voltages are obtained by frequency discriminating circuits for control purposes. No systems are known in the prior art in which the object of the lter is solely to produce a phase shift proportional to the frequency of the carrier. In reality this type of demodulay By the use of lthis system of phase detection or demodulation of Aa frequency or phase modulated carrier oscillation a universal receiver may be l' have been pointed out with claims 'appended hereto.

designed in lwhich changes in adjustment only are necessary to convert the receiver so that it will receive phase or frequency or amplitude mod- -ulated oscillations or waves.

The novel features of the present invention particularity in the The method of carrying out the present invention, the. manner in which the present invention may be carried out, and the operation thereof, will be better understood by Ythe following detailed description thereof, and therefrom when read in connection with .the attached -drawings throughout which .like reference numerals indicate like-parts, and in which: p

Figure 1 illustrates van at-presentpreferred novel arrangement for demodulating frequency, phase, or amplitude modulated signals, which arrangement includes frequency control means;

vFigures 2,'3, 4, and 5 show charts illustrating h graphically vthe relation of the currents impressed on the differential detectors producing the frequency control energy;

Fl8111`e'6` shows a modification of ment of Figure 1; i

Figures la and 1b illustrate details of lter cirthe arrange- -cuits or artificial transmission .lines and phase shifter circuits, respectively ,utilized in the re* ceiver circuits of Figs. y1 and 6; :while,' v

Referring to the drawings, and inv particular to Fig. l thereoL'A 'indicatesan absorption system4 connected"witli.aj.radio frequencyamplifier the output of'whi'ch feeds, amplified 4energy to 'I'he intermediate frequency produced by the lcombination of the radio frequency signals and the first detector 2 of a superheterodyne receiver.

nected by leads 9 to the input of a crystal filter |0. This crystal lter is of thel type disclosed more in detail in Hansell United States application No. 611,050, filed MayJl3, 1932, and in United States application #564,770 filed Sept. 24, 1931.

A crystal filter, as used here and described in the aforesaid applications, utilizes the lextreme selectivity afforded by the piezo-electric resonance of a crystal by exciting the crystal through one set of electrodes and obtaining the-piezoelectric energyfroxn another set. In this manner a selectivity is obtained which is sufficiently sharp to completely eliminate the side-bands from the modulated wave and to build up or pass readily the carrier frequency.

Preferably the crystal :filter circuit included in unit I0 includes an arrangement as disclosed in Fig. 7. This circuit comprises a transformer |00 having its primary winding connected with the line 8 and its secondary winding connected as shown by way of a terminal I0| on shield ||3 to a metal plate and to ground G and by way of terminal |02 to'a second metal plate |06.

'These metal plates |05 and |06 bear ,on a crystal |00 and are coupled therethrough to plates |01 and |08 respectively as shown. In order to insure that the only coupling between these plates is by Way ofDthe crystal partitions I|0 are formed in the metal box H3 and extend from the walls thereof to approach the crystal |09 on both sides between the metal plates |05 and |01 and |06 and |08. The crystal |09 is mounted as shown on an insulating support The metal plate |01 is connected by way of terminal |03 to the control electrode ||4 of a thermionic tube |I2. The input circuit of tube |2 is completed by connecting the cathode there; of to ground and by connecting the plate 8 by way of terminal |04 `to ground. All of the terminals |0|, |02, |03, and |04 are insulated from the box H3. Charging potentials for the anode and screen grid electrode of tube ||2 are supplied from a source H5. Biasing potential for the.

control grid |.l'4 is supplied by way of resistance ||8 from a source |I1. The secondary winding of the transformer ||6 having its primary windingconnected as shown with the output electrodes of tube ||2 may be connected as shown with the line I3 of Fig. l. In the above arrangement the coupling between the circuits due to the crystal holder are shunted out. The only coupling between the circuits is by way of the crystal |09. l

'The crystal filter may take the form of the arrangement shown iii-Fig. 8. This arrangementis somewhat to the amusement of Fig. 7

and needs no description other than to note that the lower plates |08 and |08 are -not grounded as in the arrangement of Figure 1, but are connected as shown to the high potential ends of the sec- 10i/"The operation of the filter now depends on the magnitude of the impedance of the secondary of and the impedance of |I8.4 Here as in Figure '1, the shield ||0 shields out the natural capacity of-thecrystal holders |05, |05, and |01, I08'to give the lter circuit a symmetrical characteristic. Y

The other pair of terminals of the double pole, double throw switch 1 are connected through leads I I tothe input of an artiiicial transmission line or tuned filter circuit I2 of the type disclosed more in detail in United States application No. 618,154, led lune 20, 1932.. This transmission 'line or tuned lter circuit forms no part of the present invention and Adoes not warrant a detailed description here. The lcircuit may, however, as shown in Fig. 1a, comprise series inductive reactances X and parallel capacitive reactances Z.

'The input of the line may comprise an inductance coupled to the output of the detector in 4. The line may terminate in an impedance W. Waves of the desired phase relative to the phase of the waves impressed at II may be selected in the line connected with Il by means of a point P movable on the line. Other types of transmission lines .or tuned filters may be utilized, and I do l not propose to limit my invention to any par-` ticular type of artificial line, transmission line or `tuned filter circuit. This artificial transmission line or-tuned filter circuit is for use in receiving frequency modulated oscillations. d The double throw, double pole switch 1 permits the intermediate frequency amplifier to be connected veither to the crystal filter I0 or the artificial transmission line I2, for either phase and amplitude modulated -wave reception or frequency modulated wave reception. The output circuit of the crystal filter unit III is connected with two terminals I3 of a double pole, double throw switch I4. The output circuit of the artificial transmission line I2 is connected tothe other two terminals I5 of the double pole, double throw switch I4. The blade elements I6 of the double pole, double throw switch are connected with a limiter and phase -shifter I8. The double pole,

double throw switch I4 permits the amplitude limiter and phase' shifter I0 to be connected either to the crystal filter circuit I0 or to the articial transmission line I2, depending on the manner in which the carrier frequency to beI received hvas been modulated. The amplitude limiter and phase shifter I8 contains means to malntain the filtered signal at a constant level, re-

' gardless of fading of the signal and phase shifting means, to regulate thev phase of the carrier frequency. I

part of the present invention and the phase shifter may be of any type known. For example, it may comprise. as illustrated in Fig. 1b, an artiiicial line of the type disclosed in detail in vmy U. S. appln. Ser. No. 618,154, led June 20, 1932. Briey, such a phase shifter may comprise series inductive reactances X' and parallel capacitive reactances Z. -The wave may be Vapplied to one end of the line, the other end -of which may be terminated by an impedance W. Wave energy of the desired phase relative to the phase of the applied energy may be selected from points, one of which is fixed on the low potential side of the line and the other of which is movable along the inductive reactance, as illustratedin Fig. la. As an alternative, an inductance P' may be movbly coupled to the inductive reactance in the line and may be moved along the line to a position at which the current induced in the inductance is of the desired phase relative to the phase of thefenergy impressed on the line.

The local oscillations to be introduced into the first detector 2 for producing an intermediate frequency are produced by a unit 24 including' a thermionic tube 20 having its anode 2| and control electrode 22 connected in a circuit for producing sustained oscillations comprising an inductance 25 shunted by a variable capacity 21 and a source of potential 29. The oscillations generated in the oscillation generator 20 are of a frequency determined to a major extent by- -controlling the frequency of the oscillations produced by the local source in a manner characteristic of changes in frequency of the incoming signals, thereby maintaining the receiver apparatus in tune to the modulated signals, will now be described.

The automatic frequency control unit 30 comprises two thermionlc rectier tubes 32 and 34 having their control electrodes 3| and 33 fed cophasally Iwith unfiltered signal energy through transformar secondary winding 36 connected between the control electrodes 3| and 33 of tubes 32 and 34 respectively and the cathode 31 of said tubes by way of a biasing source 38. Unltered signal is introduced into the secondary winding 36 from the primary winding 39, which is connected over the line 6 with the output of the intermediate Vfrequency amplifier 4. The tubes 32 and 34 have their control electrodes fed in phase opposition by ltered and limited signals impressed on the secondary winding 42 from the primary winding 43, which is connected with the output circuitof the amplitude limiter and phase shifter over line 44. The anode electrodes 46 and 41 oftubes 32 and 34 respectively are connected through resistances R; and R2. Charging potential for the anodes of these tubes is furnished by a source 4 9 connected between the resistances R1 and R2 and the cathode 31 of lthe tubes 32 and 34. The high potential terminals of resistance R1, R2 are connected over lines 50 to the control electrode 5| and cathode 52 of a control tube 53. The drop across the re- I rent biasing source 55', andthe series resistances 56 and 51. An inductance 66, coupled to the inductance 26 in the oscillation circuit of generator 20, is connected between .the anode 6| and cathode 52 of'modulator tube 53.

In this manner, as will appear more in detude limiter I8 are fed toa second phase shifter unit 62 and from there to unit 63, by means of which the modulated carrier frequencies are converted into audio frequency. The phase'shifter vmay be Vof any known type, such as for .example of the type discumed hereinbefore in connection with the circuits in I8. The unit 63 contains a pairof thermionic detectors 64 and .65 having their control electrodes connected to inductances 66a and 66 which form the secondary windings of transformers Ta and T having primary windings 61 and 10. Primary winding 61' is connected by atap 68 in parallel with a portion of a potentiometer resistance 68 connected with the output circuit of 62, and primary winding 10 is connected by a tap 1| in parallelwith a Apotentiometer resistance 12 connected in parallel with the first potentiometer resistance69. By this means illtered energy from the phase shifter 62 is impressed ;inphase opposition on thecontrol electrodes 13 dhd14 of tubes 64 and 65 respectively.

'I'he line 6, connected with the intermediate fre' quency' amplie'r output, is connected over line 6a to the primary winding 15 of a transformer 16' having a secondary winding 11 connected through a biasing source 118 between the control grids 13 and and the cathodes 19 of tubes 64 and65. In.this manner unfiltered energy appearing in the f output of the' intermediate frequency amplifier 4 is impreSsed'co-phasally on the control electrodes of the rectier tubes 64, 65.' `Potentiometer resistances 69 and 12 regulate the amplitude of the n ltered and limited carrier, which is fed to the detectors' or rectiers 64 and 65. The'phase of this carrier is regulated by the phase shifter 62.

The detected or demodulated signals impressed y on l64 and vproduce audio frequency oscillations onthe anodes 8| and 82 of tubes 64 and 65 respectivuely. The anode 8| of tube 64 is connected through .a ,primary winding 83 of combining transformer 86 and an anode charging potential source 84 lto the. cathode of tube' 6 4 while the anode.8 2 is connected through the primary wind-- ing 85 ortrajnsformer 80 -andthesource 84'to the' cathode 18,-. A double pole,I double throw re versing switch permits the transformer primary winding 85 to, connect the anodes vof tubes 64 and nectedwith any utilization circuit, as, for in- 65 in ,pushpull relation orv in parallel. A secondary 88A of the transformer 80 is constance,v on aural indicatoreA @we g time.` However, the problem of maintaining syn- .chronism of the signal and crystal lter hasbeen- Y flater tube 53 near enough vto the proper `r,The principle of the operation, of the ltered carrier receiver as a phase and amplitude Vmodu-v lation receiver has been known to the art of some made the receiver impractical due to the necessity of constant -retuning. In the receiver herein. de-

scribed, part of the crystal filtered energy is n iade use of to effect a constant retuning of the lnigh frequency oscillator and maintain synchro ism. In the arrangement of Fig. 1 the filtered and limitedcarrier is fed to the automatic frequency dontrol Vdetectors in phase opposition through tramsformer T2. Vectorially these two voltages to the\ two'detectors would be represented by F and Fl of Figs. 2 and 3. The unfiltered signal is fed to the detectors cophasally by transformer Ta as represented by vectors S and S1 in Figs. 2 and 3. These relative phase relations of the ltered and unfiltered signal are obtained by means of the phase shifter in unit I8. Hence, the resultant voltages presented to the two detectors are as portrayed by vectors FS and FSI. 'I'hese conditions are obtained when the signal (i. e. in the intermediate frequency amplifier) is tuned to theA peak of the crystal filter.

When the frequencyof the signal, or the high frequency oscillator, varies, the output of the crystal .filter changes very rapidly, with frequency, not only in amplitude, butalso in phase. 'I'he amplitude variations of the crystal lter output, with frequency, are removed in the limiter included in unit I8, but the-phase variations remain; hence, at a slightly different frequency the conditions portrayed byFigs. 4 and 5 exist.

i Thus, the filtered output rotates in phase so that the amplitudes oi the resultant voltages are no longer equal, but have changed in opposite directions. A change of Afrequency in the opposite direction reverses the phase change of the crystal filter output so that differential voltages are presented to the two detectors to give differential currents through resistors R1 and Rz. The voltage drop across these two resistors will have a magnitude depending upon how far off tune the signal is from the lter and a direction depending upon which side of the filter peak the signal is on.

This differential voltage across the resistors Rr and R2 is used to serve as biasing potential for the control electrode of the modulator tube 53. VConsequently the differential voltage operates to vvary the anode to cathode impedance of the modulator tube. The effect of varying the anode impedance of tube 53 is to vary the frequency of the oscillator circuit through the coupling coil 60. Thus, as soon as the signal starts to fall out of the crystal filter, the frequency of the high frequency oscillator is changed an amount sufcient to bring the frequency of the intermediate frequency wave coil to the frequency of the crystal filter. In this manner an automatic frequency control is effected which maintains the signal constantly in tune with the crystal filter.

In order that the control will hold over during a complete or partial fade-out of the signal, the time constant of the circuit consisting of condenser 54 and resistors 56, 51, R1,and Rz-is adjusted so that the-condenser will remain charged asumcient length of time to hold the biasing effect due to drop through Ri R2 on the moduuntil the signal returns.

The filtered carrier is combined with the unfiltered signal for detection in unit 63. For phase gand amplitude modulation signals the drops across the resistors R1 and R2 may be utilized for the audio -output but to make the receiver more flexible and universal, and to improve the point detection of amplitude modulation, a separate set o1.' detectors are used. With this separate detection the amplitude and phase of the carrier may be regulated at will for the proper combination with the unfiltered signal;

To adjust the receiver for reception of frequency modulation, the crystal lter I is switched out and the artificial line or tuned circuit lter lI2 switched in. The purpose of this line or lter is to produce a phase change in a frequency modulated wave proportional to the frequency of said wave. Thus, if the electrical length of the line` or' lter remains constant with frequency, the phase of its output will change with frequency. 1f this output is -then combined with the unltered signal, a change in frequency will produce a change in phase of combination ofthe two voltages and consequently vary the resultant amplitude, as set forth more in detail in United States application No. 618,154 filed June 20, 1932. This variation in amplitude is'utilized in the automatic frequency control detectors to hold the receiver in tune, and in the'audio frequencydetectors, to detect the modulation.

The operation theory of this type of phase detecticn when applied to frequency modulation proceeds'as follows: 1

The frequency modulatedwave is given by:

(1) e=Em sin (wt-i-B-fd/jm cos (ot-H0) where w is the radiation angular velocity, B is' the arbitrary phase angle of the carrier wave; fd is the frequency deviation with modulation; fm is the modulation frequency; iris thexmodulation i 'angular velocity and a is the arbitrary phase angle of the modulation frequency. The initial conditions are: Bmx- 40 when t=0. The frequency of thewave given by (l.) `differentiates to:

2) f-' f c+`fd sin opta-qi (fer-carrier frequency) In the demodulating circuit of the receiver, the unfiltered wave given by (1) is combined with a wave passed through-a device whose output phase is proportional to the frequency so' that the output phase is given by:

substituting (v2) in (3) gives the phase angle added to the wave emitted from the output of the device or:

Adding this phase angle to the phaseangle of the wave given by (1) or the input to the articial line or phase multiplying device, gives the wave emitted at the output or:'

The input of one of 4the differential detector tubesV receives the resultant of (5) plus (1) and the other tube the resultant of (5) minus '(1). Taking this sum or difference, and squaring to 4gives as the direct current terms:

.quantity (Kfc), and the other decreases.

and as the audio frequency terms:

The operating conditions make the angle (Kfc) equal to 90, 270, 450, etc., degrees so that when the signal is in tune, the direct current differential currents are equal to As soon as the signal,- falls out of tune, cos (Kfc) becomes appreciable and one detector plate current is given by:

Hence the frequency Vcontrol operated by these differential currents Vgis decreased in sensitivity as the modulation is increased. However, the limit of modulation would be reached before the frequency control lost control.

From Equation (8) .it can vbe seen that when (Kfc) is equal to 90, 270, 450, etc., degrees, the audio outputs are given by:

-tector output. Even harmonics only appear when (Kfc) is other than 90, 270, 450, etc., degrees, Since the automatic frequency control holds the value of (Kfc) at 90, 270, 450, etc., degrees, second harmonic distortion is absent from the receivers output. Odd harmonic distortion is present but only to a degree given by the Bessel function of 'the order of the harmonic; hence, at full modulation the third harmonic distortion is less than 10 per cent.

When the receiver is adjusted for phase mod- A ulation the crystal filter employed is selective enough to completely remove the modulation so that a synchronized'carrier is available. This carrier is combined with the unltered carrier wave with a 90 degree phase displacement; the direct current outputs are used for frequency control, and the audio frequency output is the received modulation. The detector outputs are given by:

(12) e=mmlsin Y(wt-H3) i- E'm sin (wt-ITP sin t+a) l2 (Pf-:phase deviation) From (12) A the direct current outputs are found to be: l

second harmonics present.

2(9St-I-a) +J4(P) COS4(t+a) +Je .COS B) :l WhenB is at its propel' operating value, namely,

90, 270, or 450, etc., degrees, the outputs of the J.3(P) sin 3(t+a z). .l Since full modulation corresponds to a value of P equal to 1.4, the percentage of thirdharmonic is less than 10 per cent for full modulation as in the case of frequency modulation. There are no der to reverse the fundamental modulation so that balanced detection may be used, the synichronized carrier is combined with the signal at one'of the detector inputs so that the resultant carrier is of opposite polarity to that present in `the unfiltered signal. For such a balanced detector scheme'the input of one detector is given by the unltered signal or:

'(16)V e=Emu+m sin cpm-ai) sin wt 'and the input to the other detector byy the combination of the unfiltered signal and synchronized carrier given by:

' The audio output of a squar/e-law detector fed while `(17) squared components:

gives as the audio frequency vand by adjusting the amplitude of the synchro- 'nized carrier so thatv 'theoutput is given by:

are added so that the fundamental components add ,and the second harmonics cancel. The out- Aput is then given by:

and no harmonics are present.

When receiving amplitude modulation, the automaticfrequency controldetectors are set the same as they are forphase modulation reception, that is, with a 90, 2'70, or 450, etc., degree displacement between the filtered and lunfiltered signal. Consequent1y,'tl1e theory of operation is the samev as the case of phase modulation.'

As mentioned inthe operation theory,l for frei quency and phase modulation reception, the autof matic frequency control detectors 32 and 3l of Fig. 1 be used also as the audio frequency detectors to supply useful current to the load circuit. This is accomplished by feeding one audio frequency amplifier :tube 90 from resistor R1 through blocking capacity C and another audio frequency amplifier tube 92 from resistor R2 through blocking condenser C1, as indicated in the modification shown in Fig. 6. The dliferential currents flowing in R1, R2 produce potentials .which are impressed across the grid electrodes 9|, 93 Vof tubes 90, 92 respectively in lDetectors 32 and 33 now perform the dualr function of producing differentialcurrents for the automatic frequency control to be fed to tube 53, andv audio frequency currents corresponding to the signal to be fed to amplifiers 90 and 92. Since the inputs to both'v types of detectors are the same, one set may be utilized to produce both the control and signal lcurrents.

The receiver of Fig. -6 andthe frequency control means therefor operates the sameas the corresponding elements of Fig. 1, and a description oi' the receiver and the frequency control means of Fig. 6 is thought unnecessary repetition.

In practice the primarywindings 96' and 91' of transformers T5 and T's may be inserted in series with the resistances R1, R2 of Figures 1 and 6, as indicatedv in Fig. 6a. 'Ihe secondary windings 96 and 91 of these transformers then may be utilized to energize the input electrodes of the tubes 90 and 92. The differential currents owing in the resistors in series with the primary windings of the transformers will furnish the differential potentials for controlling the modulator tube 53. The differential currents flowing through` the primary windings of the transformers will also in'duce potential-variations in the secondary windings of said transformers, which may be utilized in a. manner which will be clear to anyone skilled in the art by merely inspecting Fig. 6a.

My frequency control system is not limited to use with the circuit shown. The frequency control means of thev present invention may be used with any circuit wherein a frequency change may be produced by a direct current change. For instance, my control means may be applied to the dynatron frequency modulator, described in detail in United States application No. 608,383, Vled April .30, 1932 or the two tube type, of the dynatron frequency modulation, described in United States application No.

621,569 led July 9, 1932, now Patent No. 2,032,403,

March 3, 1936. Capacity in the oscillator circuit is varied by. the diierential currents in R1 and Ra.

This receiver also lends itself to the various schemes for automatic volume control. Diversity reception may be effected by using more than one receiver and adding/the audio outputs directly or by the known' forms of automatic volume control diversitfreceptlon.

Having tthus described my invention and the operation thereof what I claim is:

1. The combination with a beat frequency receiver including a local source of oscillations and a dekibdulator connected therewith, said deceiver including a local source of oscillations and said receiver output and to said crystal filter output on the one hand and to said tuning means a demodulator coupled thereto, said demodulator having an output circuit, of a crystalfilter having an input circuit connected with the output circuit of said demodulator, said crystal filter having ann output circuit, rectifylng means having an inputA circuit connected with the output circuit of said filter. said rectifylng means having an output cir'- cuit, a circuit connecting the input circuit of said rectifylng means to the output circuit of said demodulator and frequency control means connecting the output. circuit of said rectifylng me to said local source of oscillations.

3. A phase modulated signal wave receiver including circuits having variable tuning means comprising. a crystal ilter connected with the 4 output of said receiver, and circuits connected to on the other hand and responsive to unfiltered signal wave energyfrom said receiver and filtered signal wave energy/from said lter' for control- 11113V said tuning means..

4. A signal-modulatedv wavev receiving means comprising `Signal modulated .wave responsive meaus having an output,`a demodulator having anl with said demodulator, wave ltering means input and an output the input of said demodur iatbr being coupled to the output of said signalwave responsive means, an oscillator connected having an input connected with the output ,or said demodulator, said ltering means having an output. rectifylng means having an input connected with .the loutput of s aid demodulator, said rectifylng meansjhaving an output. a separate `circuit connecting the` input of said rectifylng means to the output of said`\filter, and a connection lbetween the output of said rectifylngmeans and said oscillator. ,n

5. Phase modulated signal demodula comprising aV receiver of the-heterodyne including a local oscillation circuit having a frequencydetermining reactance and a demodulator having an input circuit coupled to said oscillation r circuit, 4said demodulator having an output circuitga pair of thermionic tubes each having an anode and a control electrode, said tubes havingtheir anodes connected in push-pull relation, a circuit coupling the anodes of said tubes tolsaid reactance in said oscillation circuit, a circuit connecting the control electrodes of said tube to the output circuit of said demodulator for energizing said electrodes cophasally. by energy from the output of said demodulator, and acircult inciuding a iilter` connecting the outputcircuit of said demodulator to the control electrodes of said tubes for energizing the control electrodes of said tubes in phaseV opposition by filtered energy from the output of, said demodulator.

6. The combination of a heterodyne receiver including a local oscillator having an input cir;

cuit including frequency determining means and accasion signal absorption` means,

means an output circuit, a demodulator having an input circuit coupled to the output circuit of said local oscillator, said demodulator having an output circuit, phase changing means having an input circuit connected with the output circuit of said demodulator, said phase changing means having an output circuit. frequency control means having an output circuit coupled to the frequency determining means in the input circuit of said oscillator, said frequency control-*means having fan input circuit, a circuit connecting the input circuit of said frequency control means tothe output circuit of said phase changing means, and a separate circuit connecting the input circuit of said frequency control means to the output circuit of said demodulator.

7. In signal modulated wave receiving means, signal amplifyiiflsr means having an input circuit connected to said absorbing means; said amplifying means having an output circuit, a local oscillator including frequency determining means, a circuit connecting said oscillator'to the output circuit of said amplifying means, a demodulator having an input circuit connected to said amplifying means, said demodulator `having an output circuit. a selective ,nlter circuit havlngan input connected to the 'output of said demodulator. said lter circuit having an output circuit and being tuned topass a limited frequency band, a second signal demodulating circuit having an input-connected with theoutput of said lter circuit and con-4 nected by `a separate circuit with the output circuit of lsaid rst demodulator. means in one oi? said last named circuits for shifting the phase of the energy passed thereby relative to the phase of the energy in the other of vsaid c cuits, and a connection between the output of d second demodulating circuitV andthe frequenby determining means of said oscillator.

8. In a signal modulated wave receiving means. signal amplifying and demodulating means having an output, a local oscillation producing cirlcuit havinga frequency determining reactance,

a circuit coupling said oscillation producing circuit to said amplifying and 'demodulating means,

a selective bandpass lter having an input connected with the output of said amplifying and demodulating means, said lter having an output, rectifylng means having an' input connected bya rst circuit with the output of said' amplitying and 4demodulating meansvand by a separate oircmt with the output-or said handpass alter, a second rectifylng means having an input connected by a third circuit with the output of said amplifying anddemodulating-means and by an additional separate circuit with the output oi said bandpass lter, said. second rectifylng means having an output, a thermionic tube having an input circuit and an output circuit, means cous.

pling the output circuit of said tube to the frequency determining reactance of said oscillation producing circuit. and means coupling the input shaving a control grid and an anode. a circuit coumeans, of frequency determining means and frquencyreducing means connected with said receiving means, said frequency reducing means having an output circuit, phase shifting means connected at its input with the output of said frequency reducing means, said phase shifting means having an output circuit and being adapted to produce a phase shift in the energy passedthereby, signaldemodulating means having an input circuit connected to said receiving means and also connected by a separate circuit to said phase shifting means, said demodulating means having an output circuit, an

a control circuit connecting the output circuit o u aid demodulating means tosaid frequency determining means.

11. The combination of a heterodyne receiver including a' tunable local oscillator having an output circuit. a first detector having an input circult connectedwith the output circuit of said -local oscillator, said first detector having an output circuit, a lter having. an input lcircuit connected to the output circuit of said ilrst detector, said filter having an output circuit, phase changing means having an input circuit connected with the. output circuit of said filter, frequency control means having an output circuit connected with said tunable local oscillator, said frequency control `means having an input circuit, and separate circuits connecting the input circuit of said frequency control means to the out-v put circuit of said phase changing means and t'o the output circuit of said demodulator,

l2. A receiving means as recited in claim 8 in which phasek shifting-means is connected in a said separate circuit kcommon to. a portion o circuit and said additional separate circuit. 13. Means for signalling with phase modulated waves comprising a beat frequency `receiver including,a local oscillator and a demodulator, a`

, phase sllilttiligV Circuit connected With said 4demodulator, a pair of thermionic tubes having their anodes vconnected in push-pull relation and to said source of local oscillations, .a circuit coupled between said demodulator and the control electrodes of said tubes. for energizingthe control electrodes `of said tubes. co-phasally by energy froxnsaid demodulator, and a circuit coupled between said phase shifting circuit and the control electrodes of said tubes for energizing the control electrodes of said tubes in phase oppo sition by phase shifted'energy from said `receiver.

14. The method of' receivingphase or frequenc'yor amplitude-jf dulaffedjbscillatory energ'y including the 'tep's-4 of, beating' saidv modulated osvcillatoryjenergywithoscillations froma local source toproducemodulated oscillatory energy ofv a beat frequency, lfiltering afportion of said oscillatory' energy of beat frequency'to substantiallyv 'remove the' modulation frequencies therefromgfcombining said lter energy with unfiltered ,oscillatory energy of beat frequency iand '70' utilizing the resultant produced by said combinafrom saidl local tomtherebymcontrol" the u frequency of saidioscilla 'energy of beat 4frequency.

tion to control thefrequency of the oscillations i 15. The method ici' receiving phase vor frequency v 7 l v n' L or amplitude xmodulated signals `comprising, beating said signals with oscillations from a local source to produce a beat frequency, selectively filtering said beat frequency to obtain substantially pure. carrier ywave energy shifting the phase of said substantially pure carrier wave energy, combining said filtered and-phase shifted wave energy with unfiltered energy and demodulating said combined-energy and utilizing the resultant thereof to determine the frequency of the oscillations from .the local source to thereby maintain the beat frequency at a value. which may be selectively filtered.

16. I'he method of automatically controllingl 17. 'Ihe method of automatically determining the frequency to which a beat frequency receiver is tuned which includes the steps of, producing local oscillations, beating said oscillations with the incoming signal to produce a beat note, filtering said beat note, combining said ltered beat note with an unfiltered beat note, utilizing said combined energy to produce differential 'directr current pulsations, and utilizing said dierential direct current pulsations to determine the frequency of the oscillations produced.'

18. A device for receiving, amplifying and de- `modulating signals modulatedpn phase 6r amplitude or in frequency comprising, a signal wave responsive device, a demodulator coupled to said device, a source of local oscillations comprising a therniionic tube having its input and output electrodes connected by oscillation producing circuts incl ding an inductance, a coupling between said inductance and said demodulator, a selective lter circ including a crystal; any artificial transmission line, switching means for coupling the` input of said filter or the input of said artitlcial line to the output of said demodulator, a phase shifting circuit, switching means for coupling the input of said phase shifting circuit to the output of said crystal filter circuit or to the output ofsaid artificial line, and means for maintaining said receiver in tune comprising, a pair of thermionic tubes each having an anode and a control grid, acircuitcoupling` the control grids of said tubes in phase Opposition to the output of said phase shifting circuit, a circuit connecting the control grids of said tubes in phase to the output of said demodulator, a pair of resistances connected between the4 anodes of said tubes,V accupling tube having its input electrodes connected to said resistances by way of a time constant device. an inductance Aconnected between the out'-, put electrodes `of said coupling tube and a coupling between said last named inductance and the inductance in said oscillation generator.

` 1,9. A ldevice for receiving, amplifying and modulating signals modulated in phase or amplitude or in frequency comprising, a signal wave `responsive device, a demodulator coupled to said device, a source vof local oscillations comprising a 'thermionic tube having its input and outputelectrodesfconnected by oscillationproducing'circuits including an inductance, a coupling between said 'inductance and said demodulator, a selective filter circuit including a crystal, an artificial transmission line, switching means for coupling the input.

of said lter or the input of said artificial line to the output of said demodulator, a phase shifting circuit, switching means for coupling the input of said phase shifting circuit to the output of said crystal lter circuit or to the output of said articial line, a signal recifying device comprising, a'

necting said indicating circuit in parallel relationor in pushpull relation with the output electrodes of said tubes, and means for maintaining said receiver intune comprising, an additional pair of thermionic tubes each having a control grid and an anode, a circuit coupling the control grids of said last pair of tubes in phase opposition to the output of said phase shifting circuit, a circuit for connecting the control grids of said last tubes in phase to the output of said demodulator, a pair of resistances connected between the anodes of said last named pair of tubes, and a coupling including a time constant device' between the anode electrodes of said last named pair of tubes and Y the inductance in said oscillation generator.

20. The method of receiving signal modulated wave energy which includes the steps of, reducing the frequency of said wave energy, filtering the wave energy of reduced frequency to remove therefrom sideband frequencies, shifting the phase of the filtered wave energy of reduced frequency, producing potential variations of unlike sign characteristic 'of said wave energy of reduced frequency,l producing potential variations of'like sign characteristic of said wave energy of seduced frequency, combining the potentials of like sign in phase quadrature with the potentials of opposite sign to produce resultant energy, producing indications characteristic of said resultant energy, and controlling the frequency to which said signal modulated wave energy is reduced in accordance with the character of said resultant energy.

21. The method of receiving signal modulated wave energy which includes the steps of, reducing the frequency of said wave energy, filtering the wave energy of reduced frequency to remove therefrom sideband frequencies, shifting the phase of the filtered wave energy of reduced frequency, producing potential variations of opposite sign characteristic of said phase shifted wave energy of reduced frequency, producing potential variations of like sign characteristic of unfiltered wave d energy of reduced frequency, combining the potentials of like sign in phase quadrature with the potentials of opposite sign to produce resultant energy, and controlling the frequency to which said signal modulated wave energy is reduced in accordance with said resultant energy.

22.- I'he method f receiving signal modulated wave energy which includes the steps of producing local oscillations of a frequency which differs j from the frequency of said wave energy by an intermediate frequency, beating said local oscillations with said received wave energy to produce a heat note of said intermediate frequency, sepasign characteristic of said ltered and phase shifted wave energy, producing potential variaations of like sign characteristic of the wave energy in said other portion, combining the potential variations of like sign in phase quadrature with the potential variations of unlike sign to produce resultant energy characteristic ofthe phase relation of said combined energies. combining said last obtained resultant energies to pro- 'duce controlling impulses, and controllingr the frequency of the oscillations of said local source by means of said controlling impulses.

23. The method of receiving signal modulated wave energy which includes the steps of producing local oscillations of a frequency which differs fromthe frequency of said wave energy by an intermediate frequency, beating said local Yoscillations with said received wave energy to produce a lbeat note of said intermediate frequency, separating the energy of said beat note into two portions, filtering the energy of one of said portions to obtain therefrom a ltered wave the frequency of which is equal to said intermediate frequency, shifting the phase of the energy of said ltered wave, limiting the amplitude of the energyof said shifted and filtered wave, producing potential variations of opposite sign characteristic of said shifted and amplitude limited energy, producing potential variations of like sign characteristic of the energy in said other portion, combining the potential variations of like sign in phase quadrature with the potential vari-l ations of unlike sign to produce resultant energy characteristic of the phase relation of said comf bined energies, producing indications characteristie of said resultant energy and controlling the frequency of said produced local oscillator in accordance with said resultant energy.

24. The method of receiving signal modulated wave energy which includes the steps of producing'local oscillations of a frequency which differs from the frequency of said wave energy by an intermediate frequency, beating said local oscillations with said received wave energy to produce a beat note of said intermediate frequency, separating the energy of said beat note into two portions, filtering the energy of one of said portions to obtain therefrom a ltered wave the `frequency of which is equal to said intermediate frequency, shifting the phase of the renergy of said filtered wave, limiting the amplitude of the energy of said shifted and ltered wave, producing potential variations-of opposite sign characteristic of said filtered, phase shifted and amplitude limited wave energy, producing potential variations of like sign in phase quadrature with the potential variations of unlike sign to produce resultant energy characteristic of the phase relation of said combined energies, and controlling the frequency of the oscillations of said local source by means of said resultant energy and producing indications characteristic of said resultant energy.

25. The method of demodulating ultra high frequency'oscillatory energy which has been modulated in phase in accordance with signals which includes the steps of reducing the fr e- 4quency of said oscillatory energy without'reducing the degree of modulation thereon, dividing said oscillatory energy into two portions, producing a phase shift in the energy of one of said portions such that there isa phase quadrature relation between the energies in said respective portions, and differentially combining the energies to render the signal.

26. 'The combination with a beat frequency receiver including a local source of oscillations anda demodulator connected therewith, said deodulatorzhavingan output circuit, of a filter y having an input circuit connected with the output y.ii'uit of said. demodulator, said filter having an `utput circuit, and means, for controlling the fre- T1 "dufency of said oscillations and thereby controlj Vlin'g lthe-beat frequency supplied from said de- `l0 ,modulator to said lter, connected by a first circuit between said filter output circuit and said local source of oscillations and by a second vcircuit between said demodulator output circuit and said source of local oscillations.

27, WIn a phase modulated signal wave receiver l .having 1 a circuit lincluding a .variable tuning means, said receiver having an-output circuit, a lter circuitvhaving an input connected with the output circuit of said receiver, a circuit having an input connected'to said receiver output circuit, said circuit having an output connected to said tuning means and being responsive to unfiltered signal wave energy from said receiver, a separate circuit having an input connected to the output of said lter circuit and an output connected to said tuning means, said separate circuit being responsive to filtered energy which cooperates with the unfiltered energy of said first circuit for controlling said tuning means.

28. 'I'he .method of demodulating ultra-high `frequency oscillatory energy which has been modulated in phase in accordance with signals which includes the steps of, reducing the frequency of said oscillatory energy without reducing the de- 35 gree of modulation of said energy, dividing said oscillatory energy into twoportions, producing a relative phase shift in the energies of said portions such that there is a phase quadrature relation between the energies in said respective portions, diiierentially combining the energies, producing signal indications characteristic of said combined energies and controlling the frequency to which said oscillations are reduced in accordance with said combined energies.

29. The method of demodulating high frequency oscillatory energy which has been modulated in phase in accordance with signals which "includes the steps of, producing oscillatory energy characteristic of said phase modulated energy, dividing said produced oscillatory energy into two portions, producing relative phase shift in the energies of said portions such that there is a phase quadrature relation between the energies in 'said respective portions, differentially combining the energies of said portions, producing indications characteristic of the combined energies and controlling the frequency of the produced characteristic oscillatory energy in accordance with said combined energies.

30. In a radio receiver, a source of signal energy, a radio frequency amplifier coupled to said source, means for producing an intermediate vfrequency energy from theenergy derived from said amplifier, a detector circuit, a path extending from the output of said means to the input ofsaid detector arranged to pass both the carrier -and its associated sidebands, and means in circuit with said path including a piezo-electric` crystal tuned to the carrier for amplifying the carrier independently of the sidebanda 

